To improve fuel efficiency of engines that drive vehicles and the like, multiple cylinder engines are practically used, each of which is controlled to perform a reduced cylinder operation in which: an all cylinder operation is performed during a high load; and a part of cylinders is stopped during a low load.
In general, as the number of cylinders decreases, torque fluctuation increases due to intermittent explosions. Further, when ignition intervals of a plurality of cylinders become non-uniform, the torque fluctuation increases. Therefore, the torque fluctuation during the reduced cylinder operation tends to be larger than that during the all cylinder operation. The lower the rotation of the engine is, the more significant this tendency becomes.
To further improve the fuel efficiency, engines adopting homogeneous-charge compression ignition (hereinafter referred to as “HCCI”) combustion are practically used. However, performing the HCCI combustion in all operation regions is difficult at present. Therefore, switching combustion modes in accordance with the operation regions is being considered. One example of this is that a HCCI combustion mode is used in a low-rotation low-load range, and a spark ignition (hereinafter referred to as “SI”) combustion mode is used in a high-rotation range or a high-load range.
However, in general, the torque fluctuation by the HCCI combustion that is bulk combustion by self ignition at multipoints tends to be larger than the torque fluctuation by the SI combustion that is flame propagation combustion by spark ignition. Therefore, according to such engines, large torque fluctuation may occur in a low-rotation range where the HCCI combustion mode is used.
Further, known in recent years are vehicles in which transmission efficiency of an engine is improved by omitting a torque converter of an automatic transmission for the purpose of improving the fuel efficiency of the engine. Regarding the omission of the torque converter of the automatic transmission, for example, adopting a torsional damper instead of the torque converter is being considered. In this case, by providing the torsional damper on a power transmission path, the torque fluctuation is absorbed to some extent. However, the torsional damper typically absorbs only the torque fluctuation of preset major frequency components. Therefore, when the torque fluctuation has a plurality of frequency components, or the frequency components fluctuate, it is difficult to absorb the torque fluctuation of frequency components other than the preset frequency components, unlike the torque converter configured to transmit torque through a fluid.
According to the vehicles which adopt the reduced cylinder operation of the engine, the HCCI combustion, or the technology of the omission of the torque converter of the automatic transmission, there is a problem that torsional vibrations generated by the torque fluctuation especially in the low-rotation range are amplified by resonance of a power transmission system, and this causes vibrations and noises at respective portions of the vehicle.
To solve this problem, it is known that a centrifugal pendulum damper is provided at a power transmitting shaft. The centrifugal pendulum damper includes: a supporting member configured to rotate together with the power transmitting shaft; and a pendulum that is a mass body supported by the supporting member so as to be swingable around a point on a circumference having a predetermined radius from a center axis of the supporting member. If the pendulum swings by the torque fluctuation, component force in a circumferential direction is generated at the supporting member that receives centrifugal force acting on the pendulum. This component force serves as anti-torque that suppresses the torque fluctuation of the supporting member and the power transmitting shaft. This centrifugal force is proportional to the weight and turning radius of the mass body. Therefore, damping performance of the centrifugal pendulum damper can be improved by increasing the weight or turning radius of the mass body. However, in this case, there is a problem that the centrifugal pendulum damper itself increases in size, and this is disadvantageous in terms of its weight and arrangement space.
Further, this centrifugal force is proportional to the square of the rotating speed of the mass body. Therefore, as disclosed in PTL 1 for example, it is though that: the centrifugal pendulum damper is connected to the power transmitting shaft through a speed-increasing mechanism including a planetary gear set or the like; and with this, by increasing the rotating speed of the mass body, the damping performance of the centrifugal pendulum damper is improved while avoiding the size increase of the damper.
PTL 2 discloses an apparatus including an engagement/disengagement mechanism configured to connect and disconnect the power transmitting shaft and the centrifugal pendulum damper. To prevent noise from being generated, the engagement/disengagement mechanism cuts off the power transmission to the centrifugal pendulum damper in the low-rotation range of the engine. The noise is generated since: when the engine is operated in the low-rotation range at the time of start-up or the like, and the rotating speed of the power transmitting shaft provided with the centrifugal pendulum damper is therefore low, the centrifugal force acting on the pendulum is low; and therefore, when the torque fluctuation larger than anti-torque generated by the centrifugal force is generated, the pendulum swings to contact a peripheral member. The apparatus of PTL 2 prevents such noise from being generated.